The operation of electronic devices, such as computers, disk drives, television sets, and VCRs, is controlled by electrical circuits. These electrical circuits are packaged together into electronic components, which typically perform a function or a set of functions. The electronic components are typically mounted in an enclosed cabinet. The cabinet provides structural support as well as protection from the exterior environment. Electronic components are frequently modular and may need to be removed for maintenance, repair, replacement, or performance upgrading. To provide access to these electronic components, it is common to see the enclosed cabinet featuring access doors or removable panels. Due to limited office space or real estate, it is not uncommon to see electronic devices placed side by side and on top of one another. In such a dense and compact environment, it is very important to have front and rear access to the internal electronic components.
As electricity flows through the electronic components, heat is generated. Excessive heat can be very detrimental to the function and reliability of these electronic components. In general, cool electronic components operate more consistently and last longer, so it is important that the heat be removed so that it does not impact the performance of the electronic components or the performance of the surrounding equipment.
Heat can be removed by conduction, convection, or radiation. One of the most cost effective methods for cooling electronic components is a form of convection cooling called forced air convection. Heat removed by this method is pushed or pulled off the electronic component with moving air. The air is brought into the electronic device with a fan or blower and is directed over the electronic components. As the air passes over these components, the temperature of the air increases. This warm air is eventually expelled from the device. Ideally, this warm air is exhausted from the side opposite of the inlet to reduce the likelihood of exhaust recirculation. Exhaust recirculation is very detrimental to the cooling of the device as well as neighboring devices because the temperature of the electronic components increases linearly with the increase in inlet air temperature. Since other heat producing electronic equipment frequently surrounds the electronic device, it is beneficial that the cooling air inlet be in the front and the exhaust be in the rear of the device. Doing so also provides another advantage: the warm exhaust air doesn't blow on the individuals using the equipment. The benefits of front to back cooling is well recognized as most electronic equipment features front inlets and rear exhaust.
The amount of cooling air that travels through the electronic device is controlled by the fan or blower, which pushes or pulls the air via a multitude of rotating propellers or blades. In doing so, some acoustic noise is generated. The intensity of the noise depends on how hard the fans or blowers must work, which depends upon the amount of cooling air that is being moved and the air flow resistance of the cooling system. The amount of cooling air depends on how much heat needs to be removed from the electronic components. The air flow resistance depends on how easily air travels through the system. If the air flow path is relatively unobstructed, this resistance is small. If the air flow path is curvaceous, narrow, or filled with obstructions, the resistance can be very high.
The current trend in electronic design is towards smaller and more powerful devices. Cooling these compact devices is becoming increasingly difficult as higher power components are being placed closer to one another. To achieve proper cooling, fans and blowers must work harder. The end result may be a design that produces excessive and unacceptable acoustic noise.
Consequently, designs that reduce the amount of cooling air that is required or reduce the air flow resistance of the system are valuable and are sought after. In general, there are two design strategies to accomplish this: serial cooling and parallel cooling. Serial cooling has one inlet, one cooling path, and one exit. Parallel cooling typically has multiple inlets, multiple paths, and one or more exits.
The key advantage of a parallel cooled system is the relatively short cooling paths. These shorter lengths normally result in a lower air flow resistance which, taken alone, could allow implementation of a smaller and quieter fan or blower. Unfortunately this positive attribute can be offset by the relative high cooling flow rates inherent in a parallel cooled system. A parallel cooled system provides fresh (non-preheated) air for each electronic component. Since each path is separate, the total air flow for the system is the sum of each of the paths. If, for example, the electronic device is comprised of two arrays of disk drives in parallel that require 40 cubic feet per minute (cfm) each, the total cooling flow rate would be: EQU 40 cfm+40 cfm=80 cfm.
Despite the benefits of shorter cooling paths, a parallel cooled system's high cooling flow rate can demand, overall, larger fans or blowers. Consequently, a parallel cooled system may yield poor acoustic performance.
The key advantage of a serial cooled system is that the overall amount of cooling air is less than that for a parallel system. In a serial cooled system, the cooling air temperature becomes warmer as it travels through the system's air flow path. Consequently, the last electronic component in the air flow path will be warmer than the first electronic component in the air flow path. To counter this, serial cooled systems operate with a higher air flow per electronic component. For example, if the electronics were the same disk drives as described above, the front disk drives still require 40 cfm of cooling air. To compensate for the preheated air, the rear disk drives would require more air flow, perhaps 50 cfm. Since the air flow paths are common, the system would need to operate at 50 cubic feet per minute for adequately cooling. This is 30 cfm less than that required for the parallel cooled system. With this aspect taken alone, a serial cooled system could use a smaller and quieter fan or blower. Unfortunately this positive attribute can be offset by the relative long cooling path of a serial cooled system. The longer path normally results in a larger air flow resistance, which is detrimental to the acoustic performance of the device.
For the foregoing reasons, there is a need for a cooling system for an electronic device that allows for front to back access, front to back cooling, a small size, and quiet operation.